An electric machine has a rotating rotor and a static stator surrounding the rotor. Small electric machines often have on open air circulation through the electric machine. There is a fan positioned on a shaft of the rotor and said fan blows air through the electric machine.
Medium sized electric machines are often provided with a closed internal air circulation within the electric machine. Air is circulated within the electric machine between the rotor and the stator in a closed loop. The rotor and the stator are provided with axial air channels through which the closed air circulation is arranged. There are further air chambers at both ends of the electric machine providing a flow connection between the axial channels in the rotor and the stator. There is further a first fan for circulating the internal air. The outer surface of the stator is provided with cooling fins in order to increase the area of the stator that can transfer heat from the stator to the surrounding air. The electric machine is further provided with a third air chamber positioned axially outside one of the two air chambers. The third air chamber has an inlet opening and an outlet opening being directed in the axial direction along the outer surface of the stator. A second fan sucks external cooling air through the inlet opening of the third air chamber and blows the cooling air from the outlet opening of the third air chamber axially along the outer surface of the stator.
There are prior art medium sized electric machines provided with a closed internal air circulation between the first axial air channels in the rotor and the second axial air channels in the stator within the electric machine having four axial second air channels in the stator. The two lower second air channels in the stator are formed in connection with the support members of the electric machine and positioned symmetrically in lower corners of the cross section of the electric machine. The two upper axial second air channels are positioned symmetrically in upper corners of the cross section of the electric machine. The four axial second air channels in the outer perimeter of the stator take up quite much of the outer surface of the stator reducing the space on the outer surface of the stator that can be provided with cooling fins. This will have a negative effect on the cooling of the electric machine.
EP 1 976 098 discloses a motor cooling arrangement. The electric machine comprises a stator surrounding a rotor having a first end and a second opposite end, and first axial air channels. The stator has axial second air channels and cooling fins on the outer surface of the stator. There is a first chamber at the first end of the rotor and a second chamber at the second end of the rotor, whereby a flow path is formed between the first air channels and the second air channels through the air chambers. There is further a third air chamber at the second end of the rotor axially outside of the second air chamber. The third air chamber has an outlet opening directed axially along the outer surface of the stator. A first fan circulates air in a closed air circulation from the first air chamber through the first air channels to the second air chamber and further through the second air channels back to the first air chamber. A second fan blows external cooling air from the outside through the third air chamber and from the outlet opening of the third air chamber along the outer surface of the stator. The number of second air channels is one or more. The embodiment in FIG. 3 contains five second air channels.
U.S. Pat. No. 5,925,947 discloses a totally-enclosed type motor. The electric machine comprises a stator surrounding a rotor having a first end and a second opposite end, and first axial air channels. The stator has axial second air channels and cooling fins on the outer surface of the stator. There is a first chamber at the first end of the rotor and a second chamber at the second end of the rotor, whereby a flow path is formed between the first air channels and the second air channels through the air chambers. There is further a third air chamber at the second end of the rotor axially outside of the second air chamber. The third air chamber has an outlet opening directed axially along the outer surface of the stator. A first fan circulates air in a closed air circulation from the first air chamber through the first air channels to the second air chamber and further through the second air channels back to the first air chamber. A second fan blows external cooling air from the outside through the third air chamber and from the outlet opening of the third air chamber along the outer surface of the stator. The optimum number of second air channels is within a range of 5 to 12.